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Crop Type Influences Edge Effects on the Reproduction of Songbirds in Sagebrush Habitat Near Agriculture

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Crop Type Influences Edge Effects on the Reproduction of Songbirds in Sagebrush Habitat Near Agriculture VOLUME 9, ISSUE 1, ARTICLE 8 Knight, E. C., N. A. Mahony, and D. J. Green. 2014. Crop type influences edge effects on the reproduction of songbirds in sagebrush habitat near agriculture. Avian Conservation and Ecology 9(1): 8. http://dx.doi.org/10.5751/ACE-00662-090108 Copyright © 2014 by the author(s). Published here under license by the Resilience Alliance. Research Paper Crop type influences edge effects on the reproduction of songbirds in sagebrush habitat near agriculture Elly C. Knight 1, Nancy A. Mahony 2 and David J. Green 1 1Simon Fraser University, 2Environment Canada ABSTRACT. Extensive fragmentation of the sagebrush shrubsteppe of western North America could be contributing to observed population declines of songbirds in sagebrush habitat. We examined whether habitat fragmentation impacts the reproduction of songbirds in sagebrush edge habitat near agriculture, and if potential impacts vary depending on the adjacent crop type. Specifically, we evaluated whether nest abundance and nest survival varied between orchard edge habitat, vineyard edge habitat, and interior habitat. We then examined whether the local nest predator community and vegetation could explain the differences detected. We detected fewer nests in edge than interior habitat. Nest abundance per songbird was also lower in edge than interior habitat, although only adjacent to vineyards. Nest predation was more frequent in orchard edge habitat than vineyard edge or interior habitat. Predators identified with nest cameras were primarily snakes, however, reduced nest survival in orchard edge habitat was not explained by differences in the abundance of snakes or any other predator species identified. Information theoretic analysis of daily survival rates showed that greater study plot shrub cover and lower grass height at nests were partially responsible for the lower rate of predation-specific daily nest survival rate (PDSR) observed in orchard edge habitat, but additional factors are likely important. Results of this study suggest that different crop types have different edge effects on songbirds nesting in sagebrush shrubsteppe, and that these reproductive edge effects may contribute to observed declines of these species. Habitat managers should avoid the creation of new orchard-sagebrush habitat edges to avoid further impacts on already declining songbird populations. Effet de bordure sur la reproduction de passereaux dans des milieux d’armoises variable selon le type de culture adjacente RÉSUMÉ. La fragmentation à grande échelle de la steppe arbustive à armoises de l’ouest de l’Amérique du Nord contribue peut-être aux déclins observés des passereaux des milieux d’armoises. Nous avons examiné si la fragmentation avait un impact sur la reproduction des passereaux dans des milieux d’armoises en bordure de milieux agricoles, et si les effets potentiels variaient selon le type de culture adjacente. Plus particulièrement, nous avons évalué si le nombre de nids et leur survie différaient entre les milieux de bordure de vergers, les milieux de bordure de vignobles et les milieux d’intérieur. Nous avons ensuite examiné si la communauté locale de prédateurs de nids et la végétation pouvaient expliquer les différences détectées. Nous avons trouvé moins de nids dans les milieux de bordure que dans les milieux d’intérieur. Le nombre de nids par espèce était également plus faible dans les bordures par rapport aux milieux d’intérieur, mais ce, seulement près des vignobles. La prédation des nids était plus fréquente dans les bordures de vergers que dans les bordures de vignobles ou les milieux d’intérieur. Les prédateurs identifiés par caméra au-dessus des nids étaient surtout des serpents. Toutefois, la survie plus faible des nids dans les bordures de vergers n’a pas pu être expliquée à partir des différences du nombre de serpents ou de tout autre prédateur identifié. L’analyse par théorie de l’information des taux de survie quotidiens a montré que le couvert arbustif plus élevé dans nos parcelles-échantillons et la plus petite taille des herbacées aux nids étaient en partie responsables du taux moins élevé de survie quotidien des nids spécifique à la prédation observée dans les bordures de vergers, mais d’autres facteurs importants agissent vraisemblablement. Les résultats de notre étude montrent que les différents types de culture ont des effets de bordure variés sur les passereaux nichant dans la steppe arbustive à armoises, et que ces effets de bordure sur la reproduction participent peut- être aux déclins observés de ces espèces. Les gestionnaires d’habitat devraient éviter de créer de nouveaux milieux de bordure vergers- armoises afin de prévenir davantage d’effets négatifs sur des populations de passereaux déjà en déclin. Key Words: agriculture; conservation; edge effects; fragmentation; nest survival; sagebrush; songbird INTRODUCTION temperate breeding birds, and several types of edge effects have Anthropogenic habitat fragmentation continues to increase the been detected. The presence of edges can influence habitat choice, proportion of edge habitat in terrestrial landscapes. When edge with specialist species often less abundant in edge habitat than habitat is created, the ecological communities present are exposed away from edges (Ewers and Didham 2006). The individuals that to different biotic and abiotic processes (Ewers and Didham 2006, reside nearer to edges can have lower pairing success (Van Horn Fischer and Lindenmayer 2007). The impact of these processes, et al. 1995, Bayne and Hobson 2001), or suffer higher nest or “edge effects” (Leopold 1933) has been studied extensively in predation (reviewed by Lahti 2001, Batáry and Báldi 2004, Address of Correspondent: Elly C. Knight, Department of Biological Sciences, 8888 University Drive, Burnaby, BC V5A 1S6, Canada, ecknight@sfu. ca Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/ Stephens et al. 2004). Most of the early edge effect studies adjacent agriculture, orchard, or vineyard. First, we evaluated indicated nest predation is consistently higher near habitat edges whether songbirds in sagebrush habitat nested more frequently (Andrén et al. 1985, Paton 1994, Major and Kendal 1996). in edge or interior sagebrush habitat. We then tested whether nest However, recent studies suggest that an edge effect on nest predation was more frequent in edge habitat than interior habitat. predation is often not detected and can vary with many factors We used remote cameras placed at natural nests in the sagebrush (Hartley and Hunter 1998, Sisk and Battin 2002, Batáry and Báldi shrubsteppe to identify nest predator species and used this 2004). For example, edge effects appear to be more prominent in information to evaluate whether differences in nest predation rate landscapes with high levels of anthropogenic fragmentation could be attributed to the abundance of known nest predators. (Donovan et al. 1997, Lahti 2001). Given the number of factors that can influence edge effects, reviews suggest potential edge METHODS effects on nest predation should be studied on a regional basis (Sisk and Battin 2002, Stephens et al. 2004). Study area We studied songbirds breeding in sagebrush shrubsteppe habitat The most commonly proposed explanation for an edge effect on in the Okanagan region of British Columbia, Canada and nest predation is increased abundance of nest predators in edge Washington, USA from May to August 2011 and 2012 (Fig. 1A). habitat (Chalfoun et al. 2002, Lahti 2009, Spanhove et al. 2009). Sagebrush shrubsteppe habitat is characterized by big sagebrush However, without identification and quantification of the (Artemisia tridentata) and bunch grasses. The Okanagan is the abundance of the dominant nest predator, this explanation northernmost region of the shrubsteppe that extends across the remains speculative. Furthermore, it is difficult to understand why intermountain west of North America. Human activities exert predators might be more abundant in edge habitat without also high pressure on the sagebrush habitat in the Okanagan and understanding the life history of the dominant nest predator. approximately 35% has been converted to other land uses such as Studies have traditionally ignored predator identity because it was orchards and ground crops (USA: 38% Dobler et al. 1996, difficult to record nest predation events (Lahti 2009). Recent Canada: 33% Iverson et al. 2008). Currently, both sagebrush studies that use modern camera technology to identify nest habitat and orchard areas are being converted to vineyards to predators have had more success explaining the presence or meet increasing consumer demand for wine. absence of edge effects on nest predation. For example, an overall absence of an edge effect on nest predation can occur because predation by one species in edge habitat is offset by that of another Fig. 1. A. Sites for studying the songbird community in species in interior habitat (Benson et al. 2010, Cox et al. 2012). sagebrush shrubsteppe habitat in the Okanagan region of British Columbia, Canada, and Washington, USA. B. Study Conversion of land for agriculture is a leading cause of global site schematic overlaid on imagery of a real study site. Each habitat fragmentation (Foley et al. 2005). An edge effect on nest study site consisted of a pair of study plots: one plot adjacent predation is more often, although not always, detected near to agriculture (edge) and one away from agriculture (interior). agricultural edges than other edge types (Andrén 1995, but
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